Optical characterization or measurement of optical properties of various samples has become an important measurement and quality control technique due to its non-destructive nature. Specifically, optical characterization is commonly used to obtain the complex index of refraction, i.e., the values of n and k, as well as physical parameters of which the most important one is thickness t. Among the many types of samples whose optical properties and physical parameters can be characterized in this manner, the most important ones are samples with thin film layers. More accurate and reliable approaches to characterizing such samples are driven by an acute need in the semiconductor industry, which works with wafers covered by ultra thin oxide film layers.
For over a century now, the two traditional optical characterization techniques used in commerce and by the semiconductor industry include spectrometry and ellipsometry. Ellipsometry was introduced by P. Drude over a century ago in “Über oberflächenschichten,” Annalen der Physik 36, 1889, pp. 532-560 and pp. 865-897. Spectrometry was also introduced over a century ago. Both of these techniques are non-destructive and have in-situ capabilities. They are also well-understood and have already manifested much of their great potential to be practiced over wide spectral ranges. Still, each of these techniques also has its unique advantages and disadvantages. These limit how certain aspects of spectroscopy and ellipsometry can be combined and leveraged to improve optical characterization methods and apparatus. To better address these issues, we will first briefly review spectrometry and ellipsometry separately.